The present invention relates to integrated circuits ("ICs") fabricated on semiconductor wafers and more particularly to electrically isolating adjacent devices of ICs from each other.
Integrated circuits have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve the improvements in complexity and circuit density, i.e., the number of devices capable of being packed onto a given chip area, the size of the smallest device feature, also known as the device "geometry", has gotten smaller with each generation of ICs. Currently, devices are being fabricated with features less than a quarter of a micron across.
Increasing circuit density has not only improved the complexity and performance of ICs, but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility.
Shrinking device geometries have presented several issues to address. For example, devices on an IC are typically electrically isolated from each other. A variety of methods have evolved to accomplish this. Early bipolar ICs used depletion-region isolation between devices. These methods rely on essentially forming a reverse-biased isolation alley, or well, around each device or cell of the IC. A depletion-region isolation technique must allow for a large inactive area of the silicon surface between adjacent devices, which adversely affects IC packing densities. The consumption of chip area using this isolation technique became more pronounced as device geometries shrank and the area required for device isolation became a larger fraction of the total IC area.
Metal-oxide-semiconductor ("MOS") devices do not require the same type of isolating structure as bipolar devices, and ICs having the highest component densities are fabricated with MOS technologies. On way of isolating two adjacent MOS devices is with the local oxidation of silicon ("LOCOS") method. The LOCOS method relies on a relatively thick field oxide to be formed between devices. This thick field oxide reduces the chance that a conductive trace laying over the oxide will act as the gate of a parasitic transistor, linking one cell to another.
FIGS. 1A-1C show simplified cross sections of a silicon wafer during a LOCOS process. FIG. IA shows a silicon wafer 10 with an oxidation mask layer 12 that has been patterned to form a window 14 that exposes the silicon. The oxidation mask is typically made of silicon nitride that has been formed by a chemical vapor deposition ("CVD") process. Ions may be implanted through the window 14 into the silicon wafer to form an isolation well 16. Oxygen and water vapor diffuse very slowly through the silicon nitride layer, compared to their rate of diffusion through silicon dioxide. A pad layer 18 of silicon dioxide is frequently grown on the silicon wafer using thermal oxidation means to cushion the surface stress between the oxidation mask 12 and the silicon wafer 10.
FIG. 1B shows the wafer after the field oxide growth step. A field oxide 20 is thermally grown, usually by a wet oxidation (steam) method. About 45% of the thermal field oxide growth is downward, and 55% is upward, the resulting layer of silicon oxide being thicker than the silicon it consumes. The oxidation mask layer 12 effectively prevents oxide from growing beneath it, although lateral diffusion of oxygen and steam, including diffusion along the pad layer 18 causes oxide to grow under the oxidation mask layer 12. The wedge 22 of field oxide that grows underneath the oxidation mask has been named a "bird's beak" because of its characteristic shape. The bird's beak is a lateral extension of the field oxide 20 into the active area 24 of a device.
FIG. IC shows the wafer after the oxidation mask and pad oxide layers have been stripped. Stripping the pad oxide typically etches away part of the bird's beak oxide, and may expose a portion 26 of the isolation well 16. Subsequent processing, such as a nitric-hydrofluoric acid dip to remove stringers, such as polysilicon or polymer stringers, following a polysilicon deposition and patterning process, may remove additional amounts of the bird's beak and further expose the isolation well.
The exposed portion of the isolation edge can cause at least two problems. First, the decrease in isolation width decreases the isolation between adjacent devices. Second, the gate oxide or tunnel oxide thickness at the exposed portion of the isolation edge is thinner than elsewhere, which may cause earlier breakdown. Therefore, it is desirable to provide an isolation structure that is not as susceptible to oxide loss at the edge of the field oxide due to subsequent processing. It would be further desirable to easily adapt existing manufacturing processes to produce the desired isolation structure.
From the above, it is seem that an improved isolation structure for an integrated circuit device is highly desirable.